How does an accumulator work?

[fezster]

Most of the literature on cold water accumulators such as the GAH Coldstream and TWS mains boost show a usable capacity of 50-60%. Is this volume usable in full? Is the initial draw off at the max pressure available (that the accumulator is pre-charged to) and then slowly decreases as the accumulator runs out. Is there a percentage at which the accumulator doesn't add any benefit, despite still holding water?

And one other concept I'm struggling with - at 3.5 bar mains and 2 bar precharge (the recommended setting according to GAH and TWS), the accumulator is supplementing the mains dynamic pressure. Does that mean whichever is the highest of mains or accumulator, that's the dynamic pressure you will be getting? Is it therefore advantageous to have a higher precharge, if your mains is capable of a higher static pressure. Just wondering why 3.5/2 is the "recommended" setting from GAH and TWS?

 

[ShaunCorbs]

In simple turns it's a big expansion vessel that is set to normally the operating pressure of the system

It doesn't boost pressure it levels out flow rate giving you the same flow as let's say one tap open

You can get some that pump the acculturator upto 6 bar (so doubles your capacity at an op of 3 bar)

 

[fixitflav]

Most of the literature on cold water accumulators such as the GAH Coldstream and TWS mains boost show a usable capacity of 50-60%. Is this volume usable in full? Is the initial draw off at the max pressure available (that the accumulator is pre-charged to) and then slowly decreases as the accumulator runs out. Is there a percentage at which the accumulator doesn't add any benefit, despite still holding water?

And one other concept I'm struggling with - at 3.5 bar mains and 2 bar precharge (the recommended setting according to GAH and TWS), the accumulator is supplementing the mains dynamic pressure. Does that mean whichever is the highest of mains or accumulator, that's the dynamic pressure you will be getting? Is it therefore advantageous to have a higher precharge, if your mains is capable of a higher static pressure. Just wondering why 3.5/2 is the "recommended" setting from GAH and TWS?

When precharged the accumulator is full of air at 2 barg = 3 bara, and fills total accumulator volume. When connected to the water at 3.5 barg = 4.5 bara, the air is compressed and reduces in volume to 3/4.5 = 0.67 x total volume. So water inflow = 0.33 x total volume.
It's possible, but unlikely, that the 2 and 3.5 bar are absolute, in which case the water inflow = 1 - 2/3.5 = 0.47 x. Not sure where they get 50-60% from, maybe whoever worked it out calculated wrong!

 

[fezster]

At 33% capacity, the benefit of even a 500l accumulator seems pretty pathetic.

 

[fixitflav]

At 33% capacity, the benefit of even a 500l accumulator seems pretty pathetic.

It depends what you're trying to do with it. If it's on a hydropneumatic system with maximum say 20 pump starts/hour, it's OK for pump flow 13 m3/h, which doesn't seem too bad. Though in practice a smaller vessel would normally be used, with a minimum run timer set at 3 minutes to limit start frequency.

 

[ShaunCorbs]

It depends what you're trying to do with it. If it's on a hydropneumatic system with maximum say 20 pump starts/hour, it's OK for pump flow 13 m3/h, which doesn't seem too bad. Though in practice a smaller vessel would normally be used, with a minimum run timer set at 3 minutes to limit start frequency.

most accumulators arnt pumped

 

[fixitflav]

most accumulators aren't pumped

Depends. I was describing a system where it is pumped, as an illustration. OP said cold water accumulators so he didn't mean an expansion vessel in a boiler.

 

[ShaunCorbs]

An expansion vessel in a boiler isn't an accumulator

 

[fixitflav]

An expansion vessel in a boiler isn't an accumulator

Matter of definition (or semantics). It works the same way.
OP hasn't said what the vessel is doing, so why he thinks 33% is pathetic.

 

[ShaunCorbs]

Boosting/ equalising his flow from the mains when multi taps / outlets are in use

 

[fezster]

Boosting/ equalising his flow from the mains when multi taps / outlets are in use

Yes, precisely. This is what the GAH and TWS products are marketed as, which is why I'd find it surprising if their 60% figure was wholly inaccurate.

 

[ShaunCorbs]

It's the safe figure you shouldn't get anywhere near that but you could if you have a few problems/ things not accounted for

 

[fezster]

In this case, it's the opposite though. You want more cold water storage, not less? Or did I misunderstand what you said.

 

[ShaunCorbs]

Always go for worse case and I always over spec / oversize them

 

[fixitflav]

Boosting/ equalising his flow from the mains when multi taps / outlets are in use

Good point, I should have read the original post more carefully.
Looking at the GAH website, Model 500 has gross capacity 450 litre, maximum water content 292 litre. Minimum precharge pressure 0.5/0.8 bar (whatever that means!).
With 3.5 barg mains and 0.5 barg precharge, air volume after pressurisation = 450*1.5/4.5 = 150 litre, and water inflow = 450 - 150 = 300 litre. It says you would only use lower precharge pressure (below the standard 2 bar) if the mains pressure is low, so water content 292 litre (>60%) seems to be very much a maximum figure.

 

[fezster]

Seems quite deceptive and underhand!

 

[fixitflav]

Seems quite deceptive and underhand!

Agreed. A bit of salesmanship there!